Focused ion beams are used in forming, shaping, or altering microscopic structures, such as semiconductor devices. See for example, "Scientific American", January 1991, pages 96-101 by Jon Orloff. A focused ion beam can be directed to a very small point on a semiconductor device and then scanned, raster fashion, over a surface where material is to be removed. When an ion impinges the surface, its momentum is transferred, resulting in the removal of one or more surface atoms according to a process called sputtering. By selecting a raster pattern of a particular shape, a correspondingly shaped area of surface material can be removed. Often several successive layers of a semiconductor device are removed in a given area in order to expose and possibly sever an underlying layer.
The material removal rate, or yield (volume of material removed per incident ion or other changed particle), however, is limited by the ion current that can be concentrated into a submicron beam, which is typically no more than several nanoamperes. Therefore, the total volume of material that can be removed in a reasonable period of time by sputtering is limited. In an effort to increase material removal rates and thus decrease processing times, sputtering-enhancing gases, typically halogen-based compounds, are commonly used to chemically enhance the sputter yield. The sputter-enhancing gases are relatively stable, except in the area of the workpiece being sputtered, where they react with the workpiece and change the material removal rate from that achieved by physical sputtering alone. In addition to increasing yield, chemically-enhanced sputtering also reduces the re-deposition of previously removed material. Furthermore, chemically-enhanced sputtering causes the yield of some materials to increase while not affecting the yield or actually decreasing the yield, of other materials, creating a ratio of yields between those materials.
The ratio of yields between different materials is referred to as "selectivity", and can be either advantageous or disadvantageous, depending on the situation. For example, where it is desirable to remove a thick layer of passivation material L1 to expose, without significant destruction, a thin underlying layer of material L2, the process would be simplified dramatically by using a gas which increases the yield of L1 or decreases the yield of L2. Such a gas would reduce the processing time required and decrease the criticality of end point detection by increasing the time, relative to the total process time, that L2 can be sputtered without significant removal. However, simply reversing the workpiece materials, for example, placing a thick layer of L2 on top of L1, and sputtering with the same gas would increase the processing time and cause endpoint detection to become critical in order to avoid over-sputtering of L1. The increased uncertainty in endpoint could outweigh any time savings achieved from an increase in yield of L2, causing the chemical enhancement to actually be more of a hinderance than a help.
In the ideal focused particle beam system, a variety of gases having different yield ratios would be available to choose from depending on the material constituency of the workpiece being machined. Unfortunately, only a few gases commonly used, most of which are halogen-based, are available, limiting the material combinations that can benefit from chemically enhanced particle beam machining. Furthermore, the yield for carbonaceous materials, such as polyimide, resists and diamond, is not significantly enhanced using the existing halogen-based gases. Another disadvantage is that halogen-based compounds are hazardous and pose handling and usage problems that complicate and increase the cost of chemically enhanced machining.